This invention relates to electrical circuits designed to measure phase difference between two waveforms and more particularly to circuits wherein noise accompanying input signals is prevented from influencing modulation of the output signal.
Prior art phasemeters use a set-reset bistable multivibrator to measure the absence of coincidence between sequential pulses respectively corresponding to polarity changes by the input signals. Typically, such circuits use parallel clipper and pulsegenerator series circuits to transform the input signals into parallel pulse trains which set and reset the multivibrator. The output of the multivibrator is averaged by a filter to give an output signal dependent in value upon the phase difference as made manifest by the extent of coincidence between parallel pulses.
It is well known in the art that the inability of pulse generators to uniformily produce coincident parallel pulses and the unpredictability of a multivibrator in responding to coincident set and reset pulses causes the output of a multivibrator to erratically shift between the "0" and "1" states for successive periods, even though the input signals have remained in phase. The presence of noise aggravates this problem by causing "jitter", abrupt, spurious variations in waveform amplitude or duration. Consequently as the multivibrator output is averaged in this type of circuit, the output signal will indicate some angle of phase difference when the input signals are in-phase or slightly out-of-phase, thereby rendering the measurement unreliable.
A second disadvantage of prior art circuitry lies in the possibility that the presence of a small amount of noise will cause an extra zero-crossing in one of the input signal waveforms. An extra zero-crossing at the trailing edge of a square wave for example, will be perceived by a pulse-generator as an extra leading edge. Consequently, the multivibrator will be set (or reset) once too often, thereby giving an erroneous output signal.